The experiments described in this revised proposal are aimed at understanding the role of the GATA family of transcription factors in development. Since each member of the GATA family of transcription factors is critical for normal development, it has been difficult to survey the defects that result from absence of these factors in late embryonic development and adult life. The investigator proposes to address these putative later functions by defining transcriptional control elements for the different GATA family members and generating partially complementing alleles in YAC vectors. In addition, the development of a second and general conditional gene inactivation protocol is proposed. Finally, a significant amount of this proposal is dedicated to defining the transcriptional control elements of complex regulatory genes in mammals in a manner that permits this knowledge to be used to test specific questions about gene function in the animal.